1. Field of the Invention
The present invention relates generally to optical ring and mesh networks. More particularly, the present invention is directed to bi-directional line switched partial rings and mesh networks composed of full and/or partial rings.
2. Description of Background Art
Bi-directional line-switched rings (BLSRs) are an important class of networks. In a BLSR network, working traffic can be carried along both directions of the ring. Each optical fiber in the ring supports transmission in one direction, either clockwise (CW) or counter-clockwise (CCW). During a normal mode of operation, working traffic is routed on the shortest optical path between two nodes in the ring. Half of the bandwidth between nodes is reserved for protection traffic, what is sometimes also known as protection bandwidth or protection channel access (PCA). Since the protection bandwidth is shared amongst all of the working traffic for any single fiber failure, the amount of bandwidth needed for protection is less than for a network using dedicated protection. But, because the switch is performed at the line level instead of each path level and requires little action at intermediate nodes, the switching times can be kept below the required limit of 50 milliseconds.
BLSR networks typically are configured with the spans coupling the nodes having either two-fibers or four-fibers. In a two fiber BLSR (2F-BLSR) each span has two fibers with one fiber assigned for CW traffic and the other fiber assigned for CCW traffic. In 2F-BLSR half of the bandwidth (channels or wavelengths) in a fiber are reserved for protection traffic (1513). In general, a 2F BLSR node may combine the working and protection bandwidth with associated multiplexers (354 also know as a working/protect splitter) onto one fiber. The switching equipment attached to these multiplexers within the node behaves like a 4F BLSR with half the capacity. Further details of conventional optical BLSR may be found in SONET Bi-directional Line Switched Ring Equipment Generic Criteria, GR-1230-CORE, Issue 4, December 1998 which is hereby incorporated by reference in its entirety
In a conventional optical BLSR, the nodes do not perform wavelength conversion. Therefore, when the network element performs a line switch, the half of channels that are used for working traffic in the CW direction, must be the same wavelength as the half that is used for protection in the CCW direction. With the bandwidth arranged in this way, changing the direction of the traffic changes its traffic type from working to protect. Thus, during a failure when BLSR changes the direction of the traffic for traffic 1530 in FIG. 18 from CW to CCW (see rerouted traffic 1630 of FIG. 19). The traffic has not changed structure, but is now flowing on the protection bandwidth and can remain optical.
In a four fiber BLSR (4F-BLSR) each span has four fibers coupling each set of nodes with two of the fibers being working fibers and the other two protection fibers. Instead of splitting the bandwidth of one fiber, a whole fiber is dedicated to either working or protection. Therefore, one must change the fiber and direction of traffic to change the type of traffic from working to protect.
SONET BLSR may be deployed using an electrical switch at each node after converting the optical signal into the electrical domain (more precisely, an O-E-O (optical to electrical to optical) switch). SONET BLSR may also take the form of a so called “virtual line switched ring” as described in U.S. Pat. No. 6,654,341, filed Oct. 19, 1999 and granted Nov. 25, 2003 the entirety of which is hereby incorporated by reference.
Also, optical BLSR (O-BLSR) may be deployed using an optical switch at each node which does not require the signal to be converted into the electrical domain (e.g. O-O (optical-optical) switch), saving this extra cost and allowing for non-SONET services. The extensions discussed in this patent application apply to both the electrical and optical BLSR networks.
For O-E-O switches, the node deals with optical fiber and timeslots carried inside the fiber, and performs timeslot assignment function. For an O-O switch, O-BLSR deals with fiber and wavelengths carried inside the fiber, and performs switching at the optical level. For the invention discussed in this application, O-E-O network elements (nodes) and O-O network elements (nodes) deal with the “signals” in different formats/granularities, but use the same network construction and control principles disclosed below.
The switching equipment has at least four modes to support a BLSR network as illustrated in the diagrams of FIGS. 1c-g show bi-directional traffic with one bidirectional line with one bidirectional port at each end. These may be shown as two parallel lines of unidirectional traffic—having twice as many unidirectional ports):
(i) Normal (N) mode has the switch passing traffic directly from the client tributary Add/Drop Multiplexer (ADM) to the line access ports and vice-versa. Lower priority traffic may be carried on the bandwidth unused during normal operation. This is shown as the dotted lines in FIG. 1c. This traffic is lower priority because it is dropped during a network failure that requires the bandwidth to be used.
(ii) Ring-Switch West (RS-W) mode is used when the failed span is on the East side of this node and has the switch passing west working traffic to the west working OADM and east working traffic to the west protection OADM. The protection tributary client traffic is not used. See FIG. 1d. An optional bridging of traffic to both the protection and working line access at the same time is possible (as shown by the dotted line) in both the optical and electrical implementations. This optional bridging is useful for keeping the working line access signal alive to determine when the link is restored. If this bridging is not implemented, an out-of-band communication signaling is required.
(iii) Ring-Switch East (RS-E) mode is used when the failed span is on the West side of this node and has the switch passing east working traffic to the east working ADM and west working traffic to the east protection ADM. The protection tributary client traffic is not used. See FIG. 1e. As with switch mode (ii), an optional bridging to the original working line access is also useful.
(iv) Intermediate nodes are not directly connected to the failed span and use the Bridge (B) mode to pass protection traffic straight through the network element. This connects the working east and west tributary traffic to the working east and west line access ports. See FIG. 1f. The protection tributary access traffic is dropped. However, the protection line access traffic is passed through as shown as the dotted line in FIG. 1f. 
(v & vi) Two additional modes are also useful in an electrical implementation to recover from some additional failures: the East and West Span (S) Switch shown in. These failures include a fiber failure if this is implemented as 4 fiber BLSR or a equipment or fiber failure in the ADM that drops the working traffic, but allows the protection channel to survive. This switch mode simply puts the normal working traffic on the protection line access for the same direction (East or West) as the normal working traffic's direction as further shown in FIG. 1g. 
In electrical BLSR networks, the electrical switch matrix is an O-E-O switch that implements the above switching operations with an Add/Drop Multiplexer at a minimum, or may utilize a more general combination of time division and space switching to permit a time slot interchange and arbitrary switching of timeslots between multiple input ports and multiple output ports. Such a non-blocking electrical switch matrix has been described in Klausmeier, et al. U.S. Pat. No. 6,343,075 granted Jan. 29, 2002 and filed Oct. 26, 1999 which is hereby incorporated by reference in its entirety. Such switches are also commercially available such as the CoreDirector® intelligent optical switch sold by CIENA Corporation.
In accordance with the SONET standard, spans transfer units of information called Synchronous Transport Signals (STS). For the different optical carrier levels OC-n (such as OC-1, OC-3 and OC-12), there is a corresponding STS-n, where n is the number of STS-1 segments or time slots. Typical spans are composed of 1, 3, 12, 48, or 192 STS-1's. All SONET spans transmit 8,000 frames per second, where each frame is composed of an integer number of STS-1 segments, such as 1, 3, 12, 48 or 192.
In an optical BLSR network each node has a ring switch module (also known as a ring switch matrix) that permits at a minimum the working traffic from a source node to be redirected onto the protection bandwidth in response to a line fault or node failure. The protection traffic is directed along an alternate optical path to a destination node that avoids the defect. Low priority “extra traffic” that may in a normal mode utilize the protection bandwidth is sacrificed to ensure that higher priority working traffic is directed to its destination. A general purpose optical space-switch may be used to provide this switching function.
FIG. 1A illustrates an exemplary and conventional BLSR network 100 having nodes 101, 102, 103, and 104. Each node includes a ring switch module (not shown) to switch working traffic either optically or electrically. Working paths between two nodes, node 101 and node 103, are shown for a normal mode of operation. As indicated in FIG. 1B, responsive to detecting a line failure (e.g., a loss of signal or a degraded signal condition) between nodes 101 and 104, the nodes detecting the line failure initiate a ring switch to redirect traffic away from the failed line using the protection capacity bandwidth.
One benefit of the conventional BLSR topology is that it efficiently utilizes protection bandwidth. However, one drawback of a conventional BLSR topology is that it requires working bandwidth to be available even when it is not used. This unused bandwidth also requires unused switching equipment, unused transponders and unused transport fiber.
Additionally, another drawback of the conventional BLSR topology is that it is difficult to implement an arbitrary mesh topology in a cost-effective manner. Referring to FIG. 2A, a single ring BLSR network 280 has nodes 201, 202, 203, 204, 205, and 206. However, for a particular application it may be desirable to form direct data links between nodes 202 and 205 to improve the quality of service and/or data capacity of the network. This results in mesh topology.
Conventionally, a mesh topology may be implemented as dual full BLSRs 200, as shown in FIG. 2B. A first full (complete) BLSR 220 includes nodes 201, 202, 204, and 205. The second full BLSR 230 includes nodes 207, 203, 206, and 208. Each BLSR 220 and 230 has its own working and protection bandwidth and acts as a conventional BLSR with half the bandwidth of each span reserved for protection traffic. However, as indicated by dashed lines 250, nodes 202, 205, 207, and 208 include an element, such as an optical-to-electrical (OEO) cross-connect (not shown) to permit data to be transferred between the BLSRs 220 and 230.
Comparing the dual BLSR network 200 with a single ring BLSR network, it can be seen that a dual BLSR ring network 200 greatly increases the equipment requirements compared with a single ring BLSR 280. In the example of FIG. 2B, at least two additional nodes and two associated spans are required to implement a link between node 202 and node 205. Additionally, high data rate electrical/optical cross-connect is typically required to couple data between the rings, further increasing the incremental hardware costs.
What is desired is a flexible BLSR protection scheme that does not require bandwidth and equipment when not needed and an optical network design that combines the benefits of a mesh topology and the benefits of bi-directional line switching with reduced hardware requirements.